1. Field of the Invention
This invention relates to a method and apparatus for reducing emissions by recirculating flue gas exhausted from a pulse combustor dedicated to deliberate one-stage pulse combustion, where a portion of the flue gas is recirculated into a mixing region, an air inlet or an air decoupler.
2. Description of the Prior Art
Many different apparatuses and methods have been designed to reduce emissions from various types of continuous combustion devices. However, there is still a need for a reliable, controllable pulse combustor which is capable of reducing emissions of nitrogen oxides by effectively recirculating flue gas.
U.S. Pat. No. 4,314,444 discloses a two-stage pulse combustor which recirculates cooled combustion products. Combustion products emitted from a first combustion chamber flow through a resonance tube into a second, larger combustion chamber. After the combustion products flow through the second combustion chamber, they are cooled by a second heat exchanger and then enter a region above a section of the second heat exchanger. A portion of the combustion products are recirculated to the pulse combustors so as to dilute the combustion-sustaining gas. As disclosed by the '444 patent, a central duct positioned around a plurality of combustors is in communication with the region above a section of the second heat exchanger. The combustion products are recirculated through the central duct. The lower portion of the central duct is divided into branch portions where each branch portion corresponds to a pulse combustor. Each branch portion has a channel which leads through an aerodynamic valve and through a conduit section to an aerodynamic valve inlet of a pulse combustor.
U.S. Pat. No. 4,331,086 discloses a starved-air combustor capable of combusting a variety of fuels. An afterburner is connected to an outlet end of a combustion chamber through a duct. Hot combustion gas from the combustion chamber heats and dries fuel within the combustion chamber. U.S. Pat. No. 4,242,972 teaches recirculation of a portion of exhaust gas into the combustion chamber of a continuous combustion furnace. Combustion material and air are fed into the furnace through a feed duct, whereas recycled combustion gas and secondary combustion air are fed through inlets that protrude through a bottom wall of the combustion chamber.
U.S. Pat. No. 4,659,305 discloses a flue gas recirculation system for fire tube boilers where a minor portion of the flue gas is recirculated to a burner. U.S. Pat. No. 4,241,720 teaches a pulse combustor wherein combustion products are withdrawn by an exhaust system. U.S. Pat. No. 4,840,558 discloses a pair of pulse combustors wherein all combustion products are exhausted into an exhaust chamber at a high velocity and are then discharged directly into the environment. U.S. Pat. No. 4,640,674 discloses a pulse combustor which is capable of burning a variety of different fuels.
U.S. Pat. Nos. 4,651,712, 4,637,792, 4,569,310 and 4,488,865 generally disclose pulse combustors which have a floating valve that reciprocates in a combustion chamber in order to regulate supply of a combustible mixture. U.S. Pat. No. 3,890,084 generally discloses a method for reducing nitrogen oxide emissions from furnaces or boiler furnaces. As taught by the '084 patent, a lower burner bank is operated with low excess air and an upper bank is operated with excess air. U.S. Pat. No. 4,851,201 discloses a method for reducing nitrogen oxide emissions from various types of combustion systems and removing nitrogen oxides and sulfur oxides from an effluent stream.
U.S. Pat. No. 2,878,790 discloses an intermittent combustion boiler having a pulse-jet unit with an exhaust pipe that discharges into a convergent-divergent ejector, which first leads into a chamber, then into the atmosphere through a passage. U.S. Pat. Nos. 3,606,867 and 3,171,465 disclose pulsating or intermittent combustion systems. U.S. Pat. Nos. 3,183,895 and 3,848,408 generally teach pulse jet engines or resonance duct burners. U.S. Pat. No. 2,546,966 discloses resonant quarter-wave pulse jet engines of multiple circuit and polyphase character. U.S. Pat. No. 2,525,782 discloses a shock wave trap for multiple combustion chamber reso-jet motors which have an air inlet and discharge nozzle means in combination with a shock wave trap which is in communication with the combustion means. U.S. Pat. Nos. 2,998,705 and 3,188,804 generally disclose valveless combustors. U.S. Pat. No. 2,115,644 discloses an apparatus for reestablishing or regulating the power of explosion and combustion engines for aerial navigation, depending upon altitude variations. U.S. Pat. No. 2,748,753 discloses an intermittent combustion boiler with a combustion chamber that is supplied with liquid fuel. The combustion chamber is used for heat generation in a resonant combustion duct and the gases exhausted from the exhaust ducts collect in a common flue or manifold, from which they are sent to a chimney.
None of the above references teach flue gas recirculation in a pulse combustor, particularly a pulse combustor which is dedicated to deliberate one-stage pulse combustion.